The long-term objective of this work is to identify the immunosuppressive role of HLA-G in the prolongation and acceptance of grafted tissue, and to determine the mechanisms that modulate immune hyporesponsiveness and unresponsiveness by HLA-G toward tissue allografts. The specific goal of the studies proposed is to determine the role of the inhibitory receptors, of which HLA-G is a ligand, in modulating the function of dendritic cells and T cells. Our hypothesis is that the triggering of the inhibitory receptors modifies the functions of the dendritic and T cells, leading to the development of immunosuppressive dendritic cells with the potential to increase survival of allogeneic grafts or allow graft acceptance. To evaluate the role of inhibitory receptors, we will utilize two new strains of genetically manipulated mice generated in our laboratory. We will measure skin allograft survival and effector T cell responses in transgenic mice that express human inhibitory receptors on either dendritic cells or T cells. In Aim 1 we will determine the mechanism of alteration of dendritic cell function mediated by PIR-B inhibitory receptors. In Aim 2 we will evaluate the mechanism of induction of donor antigen-specific hyporesponsiveness mediated by ligand/inhibitory receptor modified dendritic cells. In Aim 3 we will test the hypothesis that the triggering of human inhibitory receptor ILT4 by HLA-G affects maturation/activation of dendritic cells and prolongs skin allogeneic graft survival. We will also test in vivo the role of the HLA-G and ILT2 receptor interaction in modulation of T cell responses and allograft survival. Data obtained from these studies will reveal the potential of HLA-G for the induction and maintenance of peripheral transplantation tolerance across MHC barriers by adapting the natural immunosuppressive mechanism. The knowledge gained from the proposed studies should aid in the development of novel tolerance-induction strategies for translation into the clinic for the treatment of allograft rejection, graft versus host disease, autoimmune diseases, and allergy.